Sound propagating device



Oct. 10, 1939. c. 1.. FARRAND SOUND PROPAGATING DEVICE Filed Dec. 11,1937 2 Sheets-Sheet 1 grwemm C/az'r- L. F or/"and WKW ATTORNEY Oct. 10,1939. c. L. FARRAND SOUND PROPAGATING DEVICE 2 Sheets-Sheet 2 Filed Dec.11, 1937 6 5 3 1 7 u i I M W nfl- HEW any w Q M w A TTORNEY Patented ue-meme UNITED- STATES PATENT omen I ,175,833

Larchmont,

N.Y.,aflgnorto3adio OoIporationoIAmer- -ica,aoorporatianofDelaware IApplication December 11, 1937, Serial No. 179,2:4

J 3 Claims. This invention relates to loudspeakers and particularly tothe sound radiating portion thereof.

'I'hisapplicationisadivisionoimy application Serial No. 94,072, nledAugust 3, 1936,

g inwhichthetranslatingunitdisclosedhereinis being claimed.

It is well knownin the artto allocateapart of the acoustic range oi asound reproducing apparatustoonetypeotloudspeakerand anotherpart l orparts to another type (or other yp s) of loudspeaker.

This invention relates to the type of loudspeaker useful in a so-calledtwo-way system, whereintheacousticrangeishandledby onLytwo lltypesoil,thetypeotioudspeakeroi' the present invention beingparticularly adapted to reproduce acoustic irequencies from about threehimdred cycles to the upper limit, i. e.,-

eight thousand cycles or higher. a The objects 01' the present inventioncomprise an eilicient reproduction of sound waves over a a widefrequency range, with large power. and while attaining a substantiallyuniform distribution oi. the sound waves. particularly of the higheri'requencieswhichhave a tendency to remainin a The novel features of oneembodiment of the invention will be described in the accompanyingspecification-and particularised in the appendedclaimsinconnectionwiththeaccompanying wherein:I'igurelisasectionalelevational viewoia i embodying thepresent-invention,

m throat block and is taken alongthe line 4-4 set 01' along the line Hotl ig l, and

5 l'lgureilisabottompianvlewotaportion of the annular diaphragm showingthe connection oi the vdce coil thereto.

Referring nowto l'lgs. 1 me use driving unitiot the'loudspeakercomprises a U shaped o magnetic iramel. havinga centraicore so:

'videdontheuppersuriaceoftheoorel',andthis assembly. including the frame2, core 3, and I u pole piece I, issecured together-by means oi 'coii Iis prcvided around the corel, and receives-asupplyofdirectcurrentsoastoproduceaconstant magnetic flux between thetwo separated pole pieces 4 and t. I

-As will be noted in Fig. 2, the adjacent outer l and inner edgesofthepolepieces land ,respectively, are undercut as at ll, leaving only ashort length of parallel, closely adjacent pole faces at H. 'lhespacethusprovided betweenthe paralleli'acesIi'oithepolepieceslandlaccommodates the voice or driving coil i! of anannular diaphragm it. The diaphragm II is supported belowvits inner andouter edges by means oi inner and outer concentric supporting rings IIand it, respectively. Upper conso centric clamping rings II and it clampthe edges otthe diaphragm upon the supporting rings ll andllbymeansotboltsll.

A frusto-conical member II, with a coextensive conical member ll,secured togetherand to the gs uppersurraeeoi'theinnerclampingringilbymeans of boltsllandthe centralbolt lands casing member 2i, allconcentricwith the diaphragm ",iorman-annularsumidpassagell leadingtothecentralportion oithe annulardiaso phragm il.

'lhe'a'nnularsoundpassage 2!,atthetipotthe conical member ",communicatuwith a tubulare 28 formed in an adapter 24. Thispassageltiscoextensivewithasimilarpassagell gs formed in a throat block2'.

Adapter 24 is iitted in a counter-bored portion'onthetopofthecasingmember2Landhasanannular shoulder portion 24.whichisengaged by alockingnutl'lthreadabiysecm'edtothecaaing 40 memberii. The threat block It is threaniably securedat 26 to the upper portionof adapter ll. Theupperportionoithethroatblockltis formedin a funnelshape having diversing flanges It, so asto receivethe throatportions ora plurality of horn sections 1. to N, inclusive. A suitablecelnent ll ispr'ovided to secure the throat po'rtionsoithevariomhornstogetherbe-'tweerfthe flanges II o! the throat block II.'Ihesevarloushornsareallidsnticalin shapeand size, having theirlongitudinal axes diverging from each other away from their throatportions. Thisarrangement oi nested horn sections eliminatesthewellknowndirectionalorbeameil'ect oi the higher frequencies by means ofthe aphigher frequencies, is to be distributed. in the proximatespherical surface formed by the combined mouth opening 45 thereof. Thatis, in the case of a spherical source of sound, such as a diaphragm inthe shape of a sphere, the surface 5 of which vibrates radially inaccordance with the frequency and amplitude at every point thereon,sound would be radiated through the entire frequency range, includingthe higher frequencies, in all directions. In case, however, the sourcefrom which the sound is radiated constitutes only a segmental portion ofa sphere, this uniform sound radiation would still prevail through thesolid angle subtending the segmental surfaces and the center ofcurvature thereof. This condition is approximated by the nested hornassembly 29 to 44, inclusive, by angularly disposing the longitudinalaxes thereof so as to form at their combined mouth opening a sphericalsegment from which the sound is radiated into the atmosphere. It willthus be seen that the individual sound Waves propagated in the varioushorns will have exactly the same characteristics and will be so timedthat they will unite at the combined mouth opening to form a sphericalwave front. a The combined throat opening 46 of the various horns 29 to'44, inclusive, is also in the shape of a spherical segment. Preferably,the height of the segmental surface thus formed, as indicated by thedimension A, between the single throat 30 opening of the center horn 43and the chordal line passing through the throat portions of the outerhorns 29 and (Fig. 1) is of the order of one-quarter wave length of thehighest frequency sound to be reproduced so as to prevent distortion 5and/or cancellation of a nearly planar wave front on entering thecombined throat opening 46 and emerging from the combined mouth opening45. It will be seen, therefore,-that since all of the horns are of thesame length, intersection B 40 of the longitudinal axes of the hornsalso forms the center of curvature for both the combined throat openings48 and the combined mouth openings 45. W

As will be seen in Fig. 3, the combined thrbat portion as indicated bythe dotted line 40 is rectangular in shape and opens into the soundpassage 25 of the throat block 26, which is of the same rectangularshape and size in cross-section at its upper end (Fig. 4). The passage25 and 50 the coextensive passage 23 in the adapter 24 gradually changein cross-sectional shape until'they assume a substantiallycircularcross-section adjacent the .tip of the plug I6.

. Preferably, the annular passage 22, tubular passages 23 and 25, andthe passages forming the various individual horns 29 to 44, inclusive,have an exponential rate of taper or exponential rate of cross-sectionalincrease along the" lengths thereof, increasing from the throat to themouth portions thereof, this rate being the same throughout. The size ofthese various passages is determined bythe upper and lower frequencylimits. For examplathe size of the throat opening of the annular passage22, adjacent the diaphragm I3, is determined by the amount of airvelocity change between the diaphragm chamber and the annular passage22. The limiting factor in making the'throat of passage22 as small aspossible is the frictional resistance of the air with "the walls ofpassage 22 when moving at the increased velocity and the harmonicdistortion which results when the instantaneous pressures depart'too farfrom atmospheric pressure. The

number of horns is determined by the solid angle 1 through which thesound. particularly of the tion, an annular trough 46, with relativelysmaller the flexibility at the edge to allow the annular diaphragm I3,is of the order of one-quarter wave length of the highest frequencysound intended -to be reproduced, a wave originating at the furpresentcase (Fig. 3), the horns are arranged in five vertical rows with threehorns in each row. This particular arrangement of horns, however,

may be varied to suit the conditions of the particular auditorium ortheatre in which the loudspeaker is to be placed. The length andrate oftaper of the various horns is determined by the lowest frequency soundto be reproduced and are preferably such that the smallest dimension of1 thecombined mouth opening of the various horns is on the order of onehalf of the wave length of the lowest frequency to be radiated.

. Referring to Fig. 2, it will be noted that the annular diaphragm I3,which inthe present case 15 is constructedof duralumin of the order of.002 inch in thickness, comprises, in radial cross-secannularcorrugations 41 and 48 on either side thereof. -These corrugations 41and 40 increase on trough portion 46 to be reciprocated as a rigidsection by the voice coil I2. The sound chamber or sound box .49, asdefined by the upper surface of the diaphragm I3 and the lower surfaceof g the ring I8 and member I8, closely follows the contour of thecross-sectional shape of diaphragm l3. That is, the under surfaces ofthe ring I6 and member I8, directly above the active portion ofdiaphragm I3, are similar in shape to a the cross-sectional form of theannular trough 48 and corrugations 41 and 48. However, the curvature ofthe bottom surfaces of the ring II and member I8, directly opposite thetrough portion 46, and which extend thereintmis slightly less 3 thanthat of this trough portion. The distance from the annular throatopening 55 of the annular passage 22 to the inner and outer edges of thediaphragm sound chamber 4!, or in other words, to the supported edges ofthe-.40

to be reproduced. In the present case, the dimension is about 0.4 inch.Thus, there will be no undue cancellation of a sound wave of 8000 5cycles generated in the chamber 40 due to the difference between thetime required for a wave originating at the extreme inner or outer edgeof the diaphragm I3 to enter the soundopenins II and the time requiredfor a corresponding wave generated at the central portion of the trough46 of diaphragm II to enter the same sound v H opening '55. If, forexample, this distance between the supported edges of diaphragm I! andthe opening 55 were as great as one-half the me 3 length of the highestfrequency sound intended thermost point in the chamber 4! would be outof phase with a wave originating at a point directly opposite theopening ll and therefore the two waves would cancel each other.

Referring to Figs. 2 and 6, the voice coil I2 is secured to the centralportion of the annular trough 46, directly below the annular soundpnssage 22. This construction allows the driving force, 1. e., the coilI2, to be directly in the center of the plimger section, 1. e., the 48of diaphragm I3, and also directly inlinc with the exit of sound throughthe passage 22.

As will be noted in Figs. 2 and 6. the driving 1. orvoice coil l2issecuredtothc ccntralportion of diaphragm II by means of a. cylindricalsup-e port 56 of cardboard or thelike material around which coil I2 iswound. One edge of the support 56 is serrated with alternate serrationsl1 and:

back chambers 00 and 02.

58, etc., extending on the same side, horizontally. to form a base whichmay be cemented or otherwise secured to the under surface of the annulartrough portion 4!.

Vents I! (Figs. 2 and 5) in the form of a plurality of holes arranged ina concentric path about the diaphragm it are provided in the inner polepiece 4' to communicate the inner back chamber 60, formed between theback surface of diaphragm ll, pole piece 4, and ring I, with the outeratmosphere. A second series of vents in the form of radial slots ll,provided in the diaphragm supporting ring I4 communicate the outer backchamber ll, formed by the back surface of diaphragm I I, outer polepiece 0 and ring il with the outer atmosphere. These two series of ventsthus assist the very narrow spaces between the voicecoil l2 and theadjacent inner andcuter edges ll of the pole pieces I and 4,respectively, in relieving the damping effect on thediaphragmduetothepracticallyenclosed Iclaimasmyinvention: vLanacousticdevicecomprisingapluralitycf nested horns of equal length andidentical shape, the inlets to said horns being arranged on a curveconcentric withthe curve upon which the outlets of said horns arearranged, the distance from the inlets of the center horns to a chordconnecting the inlets of the outer horns being of the order ofone-quarter of the wave length of a frequency at least as high as 8000cycles.

2. An acoustic device in accordance with claim 1 in which the area ofeach of said horns increases logarithmically i'rom their inlets to therespective outlets thereof.

3. An acoustic device comprising a plurality of nested horns of equallength and identical shape the inlets to said horns beingarranged on acurve concentric with the curve upon which the outlets of said horns arearranged, the distanoefrom the inlets of the center horns to the centerof a chord connecting the outermost, horn inlets beingof the order ofone-quarter of the wave highest frequency to be transmitted through saidhorns.

' cumn FARR-AND,

length of the go

